1. Field of the Invention
This invention relates to an open loop system and method for altering the angular position of a stepping motor with respect to a second motor and, in particular, to facsimile systems requiring the phasing of a motor at one location with respect to a motor at another location.
2. Prior Art
A facsimile system generally comprises a facsimile transmitter station for generating signals derived from a scan of a document, a receiver station at another location for forming a reproduction of the document in accordance with the sigals received from the transmitter, and a communication channel joining the two stations.
In the case of a rotary drum facsimile system, the document to be scanned is mounted on a rotating motor driven drum at the transmitter while a copy sheet is mounted on a similar rotating motor driven drum at the receiver. The document and the copy sheet must have essentially the same angular orientation with respect to a common reference when the drums are rotating to ensure that the reproduction starts at approximately the same lateral position on the copy sheet as the image on the original document. The alignment of the angular positions of the motors and, therefore, the drums is referred to herein as "phasing" or "phase synchronization."
Facsimile systems are known which have conventional pulse generating means for providing signals indicating the relative positions of the transmitting drum and the receiving drum. The transmitter is arranged to generate a phasing pulse for each revolution of its drum with the receiver for a system generating a similar phasing pulse for each revolution of its drum. The time spacing between the phasing pulse from the transmitter and the phasing pulse from the receiver or vice versa is a function of the displacement from the desired phase relationship. Coincidental pulses or pulses having a predetermined time displacement indicate the in-phase relation of the transmitter and receiver drums.
U.S. Pat. No. 3,553,370 to Bernard M. Rosenheck discloses an open loop phasing system for a facsimile transmitter and receiver which utilizes such phasing pulses to control a pulse counter to obtain a count representing the phase difference or phase error between the rotating drums of the transmitter and receiver. The count, when initiated by a phasing pulse from the receiver, represents the amount the receiver drum leads the transmitter drum. The count is obtained at a frequency which is directly proportional to the speed of rotation of the transmitting and receiving drums. The counter is set for a predetermined count corresponding to the count pulses that would be applied during the time interval of one revolution of the drum. The predetermined count minus the count obtained represents the amount by which the receiver drum lags the transmitter drum. After the count representing the phase error is obtained, the count is continued at a lower frequency until the predetermined count is reached. The count pulses provided at the lower frequency are also applied to operate a phase correction device at the same time, so that the phase correction is completed when the counter reaches its predetermined count. The phase correction device is a stepping motor which is connected to rotate the stator of the drive motor for the receiving drum. This causes the rotor to be offset in the direction of rotation and by the additional amount of stator rotation for advancing the receiver and correcting the degree to which it lags the transmitter.
While this prior art arrangement provides for an open loop phasing system, it requires an additional motor and coupling to the stator of the receiver motor to effect the phase correction.